> I was told, but have not been able to confirm, that only 6 of the 8
> conductors in cat 5 wire are used for ethernet communications.  I'd
> like to use these spare wires for a PIC based project

Both the 10baseT and 100baseTX specifications use only 2 pair
of the 4 pair in a standard UTP (unshielded twisted pair) cable.
This applies to both category 5 (10baseT, 100baseTX) or category
3 (10baseT) wiring.

The connector usage on RJ45 jacks and plugs is as follows:

        1 2 3 4 5 6 7 8     <-- pin number
      +-----------------+
      | ! ! ! ! ! ! ! ! |
      | ' ' ' ' ' ' ' ' |   RJ45 (looking into the jack)
      |                 |
      +---+         +---+
          +-+     +-+
            +-----+

Ethernet uses twisted pairs on pins 1 & 2 and 3 & 6.  EIA wiring
scheme of wire color to pin number is as follows:

        pin     T568A           T568B
        ---     ------------    ------------
         1      white-green     white-orange
         2      green-white     orange-white
         3      white-orange    white-green
         4      blue-white      blue-white
         5      white-blue      white-blue
         6      orange-white    green-white
         7      white-brown     white-brown
         8      brown-white     brown-white

Some Ethernet patch cables are manufactured with only 2 pairs
(4 wires) on pins 1, 2, 3, & 6.
                                                Lee Jones

P.S. If you run into FDDI on copper, it uses the same connectors,
     same cable, and still uses only 2 pairs but the signals are
     on pins 1 & 2 and 7 & 8.